For an extra layer of flavor, try grilling the tomatillos instead of broiling them. If you prefer a milder heat, remove the seeds from the chile de arbol peppers before toasting. These tacos are also delicious with a dollop of vegan sour cream or a squeeze of lime juice. Feel free to experiment with different types of mushrooms, such as shiitake or oyster mushrooms. Store leftover mushroom filling in an airtight container in the refrigerator for up to 3 days.